The crosslinking of resins containing hydroxyl functional groups with amino-plast resins are known to produced good quality thermosetting plastic coatings on metal. It is also known that an acid such as p-toluenesulfonic acid can be used as a crosslinking catalyst.
Robert Saxon et al., J. of App. Polymer Sc., 8, pp. 475-488 (1964), reported the use of p-toluenesulfonic acid (p-TSA) as a curing catalyst for HMMA alone or in combination with acrylic polymers. It was found that although curing can take place without the presence of an acid catalyst, the rate of cure is very slow and the curing temperature required is very high. The presence of p-TSA lowered the curing temperature and increased the cure rate. However, resin compositions containing p-TSA is unstable at room temperature and required mixing of the ingredients immediately prior to use. This makes it difficult to control the quality of the resin produced and to put together a stable one package coating system.
Grant et al., U.S. Pat. No. 3,842,021 and Parsons et al., U.S. Pat. No. 3,868,338 described epoxy blocked p-TSA as a catalyst for the curing of polyster-HMMA resins. The catalyst is prepared from p-TSA wherein the acid group is blocked with a compound containing an oxirane group, specifically a bisphenol A epichlorhydrin epoxy resin. The maximum molecular weight achievable using such a process is about 700.
Sulfonic acid blocked with other groups such as 2-hydroxy-cycloalkyl or aryl substituted 2-hydroxy cycloalkyl groups (U.S. Pat. No. 4,469,832); alpha-hydroxy carbonyl groups (U.S. Pat. No. 4,510,290); oxa-aza cyclopentanes (U.S. Pat. No. 4,200,729 and U.S. Pat. No. 4,251,665); beta-hydroxy free groups (U.S. Pat. No. 4,454,274 and U.S. Pat. No. 4,477,618) and beta-hydroxy carbonyl groups (U.S. Pat. No. 4,504,372) have been reported.
Catalysts for low temperature cure of amino crosslinked coatings can be found also in American Cyanamid Bulletin # CRT-159 (1978).
All of the blocked sulfonic acid ester catalysts described result from an equimolar reaction of the blocking group and the sulfonic acid group. Thus, none of these are polymeric and are of low molecular weight, less than about 700. Moreover, many of the blocked sulfonic acid catalysts reported are not stable and cannot be stored for long periods of time. Compounds with amino and alcohol groups have been used to improve storage stability. However, resins cured with amine blocked sulfonic acid catalysts are less impact resistant, and have poor resistivity. The resins cured with amine blocked sulfonic acid catalysts also have a tendency to wrinkle on cure and give variations in gloss. Further, coating systems containing amine blocked sulfonic acids also has low resistivity and are, therefore, difficult or cannot be used in electrostatic spray applications, which are particularly important in automotive and appliance coatings, where high transfer efficiencies are required.
Currently, many countries require toxicity testing for new compounds. In particular, under present regulatory schemes, U.S. and European countries require toxicity testing of compounds with molecular weights of less than about 1000; whereas, polymers with molecular weights of about 1000 or greater are exempt from such testing. Thus, toxicity testing would be required even for new blocked sulfonic acid catalysts made from known processes. Toxicity testing is very expensive and is thus detrimental to the development of improved curing catalysts, since catalysts are generally used in small quantities and the expense for such testing is not justifiable.
It is, therefore, an objection of the present invention to develop curing catalysts that are polymeric with molecular weights of about 1000 and higher.
It is another objective of the present invention to develop polymeric curing catalysts that are storage stable.
Another objective of the present invention is to provide one package stable resin coating systems containing the polymeric curing catalysts which can used in electrostatic spray applications.
It is a further objective of the present invention to develop polymeric curing catalysts containing resin compositions that are storage stable and and can be used to provide cured thermosetting coatings which are superior in hardness, impact resistance, resistivity, with improved adhesion, blister resistance, salt spray characteristics and flexibility.